Fix SF bug #482171: webchecker dies on file: URLs w/o robots.txt
Fix SF bug #482171: webchecker dies on file: URLs w/o robots.txt

The cause seems to be that when a file URL doesn't exist,
urllib.urlopen() raises OSError instead of IOError.  Simply add this
to the except clause.  Not elegant, but effective. :-)

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator